> Yet we haven't heard about: d) non-theist scientists who feel that
> evolution (& science) indicates nothing about the presence or
> absence of purpose in the universe.

I suspect that is partly because such scientists are mostly busy with their
science. Those who mistakenly think they can extrapolate their science into
metaphysical conclusions are willing to put such conclusions in their
writing, whereas those who recognize that one can't get metaphysics from
science would tend not to mention metaphysics. So they may well be the silent
majority.

It is worth mentioning that the recent book by Stephen J. Gould, "Rocks of
Ages," seems to take a position that fits in your category (d). I can also
quote G. Brent Dalrymple from his book "The Age of the Earth":
"Let us be clear about one thing, however: this information provides us no
answers to the larger question of whether we are ultimately the result of a
grand and purposeful design, or merely an accident of past and current
physical processes. Science can attempt to determine how and when the Earth
and its surroundings were created, but the question of why it all exists is
not one that science can speak to" [p.3]

I do think Tim has hit upon a useful classification scheme -- one that
recognizes the core metaphysical assumption (that the natural explanations of
evolution imply no God, which is a good example of the "God of the Gaps"
fallacy) Phil Johnson et al. has in common with Dawkins et al.